1. Field of the Invention
The present invention relates to a phase comparing circuit, and more particularly to a phase comparing circuit for use with a phase-locked loop.
2. Description of the Prior Art
A phase-locked loop circuit is used for generating an output signal that is synchronized with a pilot signal having a known frequency. For example, a phase-locked loop circuit is used in the stereo demodulating circuit of an FM receiver to generate a subcarrier signal of 38 KHz synchronized by a pilot signal of 19 KHz. A phase-locked loop circuit typically includes a phase comparing circuit that compares the phases of the phase-locked output signal, or a multiple of it, and of the pilot signal to keep those two signals synchronized. The phase comparing circuit provides a control signal that is amplified and used to control a voltage-controlled oscillator that generates the output signal.
With known phase-locked loop circuits it has proved difficult to incorporate the phase-locked loop in an integrated circuit. In one known phase-locked loop circuit, transistors in the phase comparing circuit require base currents which, to provide a sufficiently small resistance in the transistors when they are on, must be so large that the integrated circuit cannot prevent current leakage between the various integrated circuit elements. Providing a sufficiently large loop gain makes it necessary also to provide the pilot signal at a high enough current level to suppress the DC offset at the input terminal of the amplifier used in the phase-locked loop, and that leads to the same current leakage problem.
An alternate configuration for a phase-locked loop circuit is available to overcome those problems. However, that configuration uses a pair of resistors which must be provided with almost exactly the same resistance. The formation of resistances in integrated circuits to the accuracy required to effectuate this solution is difficult. To overcome that deficiency, a separate control must be used to equalize the resistances. But providing a separate control defeats the purpose of forming the phase-locked loop on an integrated circuit.